[Darwin] Make a final check on PIC options.

[Iain Sandoe]

In some cases the cascaded settings for pic/pie/PIC/PIE can cause a confusing
scenario for Darwin where "fPIC fno-PIE”  produces a wrong code generation 
setting.

So we make a final check on PIC options; for Darwin these are not dependent on
the PIE ones, although PIE does require PIC to support it.  Specifically, for 
Darwin,
"fPIC fno-PIE" should result in the same as "-fno-PIE -fPIC”.

tested on x86-64-darwin16,
applied to mainline
Iain

2019-07-07  Iain Sandoe  <i...@sandoe.co.uk>

        * config/darwin.c (darwin_override_options): Make a final check on PIC
        options.

diff --git a/gcc/config/darwin.c b/gcc/config/darwin.c
index 02bcf01..5ac0925 100644
--- a/gcc/config/darwin.c
+++ b/gcc/config/darwin.c
@@ -3241,6 +3241,8 @@ darwin_override_options (void)
       && write_symbols == DWARF2_DEBUG)
     flag_var_tracking_uninit = flag_var_tracking;
 
+  /* Final check on PIC options; for Darwin these are not dependent on the PIE
+     ones, although PIE does require PIC to support it.  */
   if (MACHO_DYNAMIC_NO_PIC_P)
     {
       if (flag_pic)
@@ -3249,9 +3251,11 @@ darwin_override_options (void)
                 " %<-fpie%> or %<-fPIE%>");
       flag_pic = 0;
     }
-  else if (flag_pic == 1)
+  else if (flag_pic == 1
+          || (flag_pic == 0 && !(flag_mkernel || flag_apple_kext)))
     {
-      /* Darwin's -fpic is -fPIC.  */
+      /* Darwin's -fpic is -fPIC.
+        We only support "static" code in the kernel and kernel exts.  */
       flag_pic = 2;
     }